The number of people who depend upon a wheelchair for mobility increases as medical science continues progress in the treatment of the elderly and the disabled. Manually driven wheelchair design has evolved to the point that currently available products are lightweight, simple and reliable. The high percentage of manually driven wheelchairs use pushrim propulsion in which the user applies force to a pushrim that is provided for each main wheel.
A concern with the use of pushrim propulsion is that the poor biomechanics too often results in ineffective propulsion, pain and injury. Users of pushrim manual wheelchairs may suffer from Repetitive Strain Injuries (RSI) of the wrists and shoulders. Too often, this suffering by wheelchair users leads to downward spirals of physical activity, health and social interaction.
The shortcomings of pushrim wheelchairs cause many users to turn to electric wheelchairs. However, such wheelchairs are expensive and difficult to transport. More importantly, electric wheelchairs result in an almost total loss of therapeutic physical exertion. No matter how sophisticated the design, the health problems associated with a sedentary lifestyle result. The overall physical health, social integration and emotional well-being of users suffer.
It has been shown that lever propulsion avoids the ergonomic and efficiency shortcomings of the pushrim design, as well as the economic and sedentary concerns of the electric wheelchair design. For a pushrim wheelchair, there is only a push motion, since no energy is derived from the return stroke of a user's arm. In addition, there is only a short “window” during which the propulsion stroke is optimized, due to the significant change in geometry of the arm and its relationship with the pushrim during the push stroke. Time is also lost as a consequence of the requirement to engage and disengage the pushrim. It has been determined that the cumulative effect is that only 20 percent of the total cycle time is utilized for pushrim propulsion.
In order to maintain an acceptable speed, the required forces applied to a pushrim during the short propulsion window are high, leading to overload and subsequent injury. These forces may be magnified several fold while propelling up an incline. There is also typically a momentary negative application of force at the beginning and end of a stroke, while engaging and disengaging the hand relative to the pushrim. The end result is a significant biomechanical efficiency of less than 10 percent.
In comparison, the nature of lever propulsion encourages a steady application of force. Approximately 50 percent of the total cycle time is utilized for propulsion of many wheelchairs that utilize levers. There is less change in the geometry of the arm relative to the lever during a stroke. Additionally, the hand and lever remain in engagement. These factors alone result in an estimated 80 percent reduction in the peak required force during the propulsion cycle.
Knowledge of the structure of a human shoulder plays an important role in comparing pushrim propulsion to wheelchair lever propulsion. A shoulder is extremely flexible, as compared to a hip joint. This requires the muscles, tendons and ligaments of the shoulder to stabilize the joint when force or torque is applied through the joint. Ideally, the force applied to a hand should pass along a line radial to the shoulder joint, so that minimal torque is generated at the joint. However, for optimal mechanical efficiency, the force applied to the pushrim should be tangential to the rim, causing a considerable torque at the shoulder joint. Users of pushrim wheelchairs tend to compromise and apply a force that is closer to the center of the drive wheel. This is also required to generate a sufficient friction force between the hand and the rim. The compromise, along with the high peak loading, results in muscle, tendon and ligament strain, as well as a high loading of the joint cartilage. In comparison, lever use results in a force that is essentially in line with the shoulder joint, so that minimal torque is generated and the force applied is in the same direction as the lever motion. Motion is nearly in a horizontal direction, which more fully utilizes larger muscles, such as the latissimis dorsi, pectoralis and trapezius muscles.
Wrist mechanics must also be considered. For pushrim propulsion, the forces at the hand are not in line with the wrist, and therefore require counteracting torque. In addition, there is a counterproductive torque produced as a consequence of grasping the pushrim at the palm and index finger. The hand is required to follow the circular motion of the rim, which requires the wrist joint to flex considerably. These various elements may induce Carpel Tunnel Syndrome. For lever propulsion designs, there is no repeated flexing and unflexing of the fingers. Additionally, the required grip force is significantly reduced, since the force applied to the lever is perpendicular to the contact area. Lever propulsion significantly reduces and sometimes eliminates the factors that lead to Carpel Tunnel Syndrome.
Wheelchairs that utilize lever propulsion are known. U.S. Pat. No. 4,560,181 to Herron describes a wheelchair and drive mechanism powered by reciprocating operation of a lever. The drive mechanism provides a variable gear ratio for operation at various speeds and on different inclines. Additionally, connecting arms are coupled to the lever to alternately engage and disengage a ratchet wheel, so that energy is transferred during both a forward and a rearward stroke of the lever. Wheelchairs that utilize lever propulsion and enable both forward and rearward drive are also available. U.S. Pat. No. 6,893,035 to Watwood et al. describes a transmission between a lever arm and a wheel, with the transmission being biased into either a forward direction or a reverse direction. U.S. Pat. No. 6,017,046 to Markovic sets forth a wheelchair drive system with a number of capabilities, including selection of wheelchair drive direction, continuous transfer of power in either direction of lever movement, and hand-controlled braking. Two other lever propulsion wheelchairs of interest are disclosed in U.S. Pat. No. 6,820,885 to Oshimo and U.S. Pat. No. 5,167,168 to Beumer.
While prior art lever propulsion wheelchairs operate well for their intended purposes, further improvements are sought. One area of concern is geometry related. The addition of the lever and its transmission may add significantly to the width of the wheelchair. Changes in legal requirements and in societal perceptions have brought improvements with regard to allowing access to public areas by persons in wheelchairs, but the added width of lever propulsion may prevent maneuverability through tight spaces. The width of a wheelchair may also be an issue for storage, such as when the wheelchair is placed in a car or other vehicle. A related concern is the placement of the levers. Levers which are outboard of the wheels expose the user's hands and knuckles to collisions. An object of the invention is to provide a wheelchair drive mechanism that utilizes lever propulsion without a large increase in wheelchair width or weight. Preferably, this is achieved while enabling multiple lever-controlled capabilities.